1. Field of the Invention
The present invention relates to multi-carrier transmission systems in which the effects of cutting in the reception and sampling of multi-carrier signals is reduced, a transceiver and a receiver for use with such multi-carrier transmission systems, and a method, for use in such multi-carrier transmission systems, of reducing cutting in the reception and sampling of multi-carrier signals.
2. Discussion of the Background
The demand for provision of multi-media and other bandwidth services over telecommunications networks has created a need to transmit high bit rate traffic over copper pairs. This requirement has led to the development of a number of different transmission schemes, such as, ADSL and VDSL. One of the more likely modulation systems for all these transmission schemes is a line code known as DMT (discrete multi-tone), which bears some resemblance to orthogonal frequency division multiplex, and is a spread spectrum transmission technique.
In discrete multi-tone transmission, the available bandwidth is divided into a plurality of sub-channels each with a small bandwidth, 4kHz perhaps. Traffic is allocated to the different sub-channels in dependence on noise power and transmission loss in each sub-channel. Each channel carries multi-level pulses capable of representing up to 11 data bits. Poor quality channels carry fewer bits, or may be completely shut down.
Because inter pair interference in copper pair cables is higher where data is transmitted in both directions, i.e. symmetric duplex, a number of transmission schemes have proposed the use of asymmetric schemes in which high data rates are transmitted in one direction only. Such schemes meet many of the demands for high bandwidth services, such as, video-on-demand but, in the long term, symmetric duplex systems will be required.
VDSL technology resembles ADSL to a large degree, although ADSL must cater for much larger dynamic ranges and is considerably more complex as a result. VDSL is lower in cost and lower in power, and premises VDSL units need to implement a physical layer media access control for multiplexing upstream data.
Four line codes have been proposed for VDSL:
CAP; Carrierless AM/PM, a version of suppressed carrier QAM, for passive NT configurations, CAP would use QPSK upstream and a type of TDMA for multiplexing (although CAP does not preclude an FDM approach to upstream multiplexing);
DMT; Discrete Multi-Tone, a multi-carrier system using Discrete Fourier Transforms to create and demodulate individual carriers, for passive NT configurations; DMT would use FDM for upstream multiplexing (although DMT does not preclude a TDMA multiplexing strategy);
DWMT; Discrete Wavelet Multi-Tone, a multi-carrier system using Wavelet Transforms to create and demodulate individual carriers, DWMT also uses FDM for upstream multiplexing, but also allows TDMA; and
SLC; Simple Line Code, a version of four-level baseband signalling that filters the base band and restores it at the receiver, for passive NT configurations; SLC would most likely use TDMA for upstream multiplexing, although FDM is possible.
Early versions of VDSL will use frequency division multiplexing to separate downstream from upstream channels and both of them from POTS and ISDN. Echo cancellation may be required for later generation systems featuring symmetric data rates. A rather substantial distance, in frequency, will be maintained between the lowest data channel and POTS to enable very simple and cost effective POTS splitters. Normal practice would locate the downstream channel above the upstream channel. However, the DAVIC specification reverses this order to enable premises distribution of VDSL signals over coaxial cable systems.
When a multi-carrier signal is received, it is sampled and digitised before FFT processing, (or wavelet transform processing). Multi-carrier signals consist of many narrow band carrier waves which convey data via a wide band channel. The amplitude distribution of the sum of all the individual carriers is Gaussian. This means that high amplitudes occur with low probability. At the present time, analogue to digital convertors have a limited dynamic range. It is, therefore, normal practice to provide a compromise between cutting and quantisation noise. This means that the adverse affects of a coarse quantisation have to be balanced against signal distortions caused by loss of the low probability high amplitude signals.
The present invention provides an alternative by detecting the occurrence of high amplitude signals and estimating the value of such signals by statistical means, and using the estimated signal value instead of the saturated value derived from the analogue to digital convertor. This technique minimises the effects of cutting, caused by saturation of an analogue to digital convertor operating with a limited dynamic range, and acceptable quantisation intervals.
It is an object of the present invention to provide a method of reducing cutting in analogue to digital conversion of a multi-carrier signal while operating with a limited dynamic range and maintaining acceptable quantisation intervals.
It is a further object of the present invention to provide a receiver for the reception of multi-carrier signals in which cutting is reduced in the analogue to digital conversion of a received multi-carrier signal while operating with a limited dynamic range and maintaining acceptable quantisation intervals.
It is a yet further object of the present invention to provide a transceiver having a receiver for the reception of multi-carrier signals in which cutting is reduced in the analogue to digital conversion of a received multi-carrier signal while operating with a limited dynamic range and maintaining acceptable quantisation intervals.
It is a still further object of the present invention to provide a multi-carrier transmission system having at least two transceivers, each including a receiver for the reception of multi-carrier signals in which cutting is reduced in the analogue to digital conversion of a received multi-carrier signal while operating with a limited dynamic range and maintaining acceptable quantisation intervals.
According to a first aspect of the present invention, there is provided a receiver, for use with a multi-carrier transmission system, said receiver including digitising means for sampling and quantising a received signal comprising a plurality of individual carriers, and processing means for digitally processing said sampled and quantised received signal to extract data modulated on said plurality of carriers, characterised in that said digitising means includes an analogue to digital convertor for converting said received signal to a series of digitised samples, each sample having n bits, an extremal detector for detecting when an analogue signal at an input to said analogue to digital convertor has an amplitude greater than that which can be represented by n bits, and generator means controlled by said extremal detector to generate m bits to represent each digitised sample, where m greater than n.
Said digitising means may be operative to minimise cutting of said digitised signal resulting from saturation of said analogue to digital convertor.
A number represented by said m bits may be determined by estimation from a Gaussian distribution of amplitudes.
Said received signal may be passed through analogue amplitude regulation means before passing to said input of said analogue to digital convertor.
Said generator means may be a 3-to-1 multiplexor having a first input arranged to receive an n bit output from said digital to analogue convertor, a second input arranged to receive an estimate of a positive amplitude value representative of an amplitude of a sample of said received signal greater than a positive saturation value of said analogue to digital convertor, a third input arranged to receive an estimate of a negative amplitude value representative of an amplitude of a sample of said received signal less than a negative saturation value of said analogue to digital convertor, and an output for an m bit signal representative of a predicted amplitude of said input signal.
Said multi-carrier transmission system may be an orthogonal frequency division multiplex radio transmission system.
Said multi-carrier transmission system may be a DMT transmission system.
Said multi-carrier transmission system may be a DMT based VDSL system.
A digitised output signal from said digitising means may be passed to an input of a FFT processor means.
A digitised output signal from said digitising means may be passed to an input of a wavelet transform processor means.
According to a second aspect of the present invention, there is provided a transceiver, including a transmitter and a receiver, characterised in that said receiver is a receiver as setout in the preceding paragraphs.
According to a third aspect of the present invention, there is provided a multi-carrier transmission system, characterised in that said multi-carrier transmission system includes two transceivers as set out in the preceding paragraph.
According to a fourth aspect of the present invention, there is provided a method of digitising an analogue multi-carrier signal comprising a plurality of individual carriers each of which is modulated with a data stream, in which said signal is sampled and quantised, characterised by the steps of:
sampling and quantising said analogue multi-carrier signal to produce a sequence of digital samples each of which contains n bits representing said sample amplitude;
detecting when an analogue multi-carrier signal sample has an amplitude greater than the maximum amplitude that can be represented by n bits; and
generating a digital signal in which each sample is represented by m bits, where m greater than n.
Cutting of said digitised signal resulting from saturation of an analogue to digital convertor used for said sampling and quantization may be minimised.
A number represented by said m bits may be estimated from a Gaussian distribution of amplitudes.
Said analogue multi-carrier signal""s amplitude may be regulated prior to sampling and quantization.
A positive amplitude for said analogue multi-carrier signal, greater than the positive saturation value of said analogue to digital convertor may be estimated, a negative amplitude for said analogue multi-carrier signal, less than the negative saturation value of said analogue to digital convertor may be estimated, and:
if a sample of said analogue multi-carrier signal has an amplitude lying between said positive and negative saturation values, an output of said analogue to digital convertor may be used to represent the amplitude of said sample of said analogue multi-carrier signal;
if a sample of said analogue multi-carrier signal has an amplitude greater than said positive saturation value, the estimated positive value of said amplitude may be used to represent the amplitude of said sample of said analogue multi-carrier signal; and
if a sample of said analogue multi-carrier signal has an amplitude less than said negative saturation value, the estimated negative value of said amplitude may be used to represent the amplitude of said sample of said analogue multi-carrier signal.
Said multi-carrier signal may be an orthogonal frequency division multiplex signal.
Said multi-carrier transmission signal may be a DMT transmission signal.